A method of using a radio signal as a backhaul line has attracted attention in recent years. In a communication device that forms a communication line by such a radio signal, it is necessary to optimize reception power by using a reception power monitoring device such as a level meter that monitors reception signal power for adjusting the antenna orientation. The level meter generally drives with an external power supply such as a battery. However, because battery maintenance is required for the reception power monitoring device despite that it is used for installation, maintenance and inspection of a radio device and the like and not used all the time, there is a problem that the management cost is high.
In view of the above, the technique that generates power for a device or a circuit to operate from a radio signal is disclosed in Patent Literature 1. According to Patent Literature 1, a data transmitting line that is connected to a pulled-up data transmission line of an external device is provided with a power supply conversion means that converts electric energy on the data transmitting line into voltage and supplies it to a main control device.
Further, the technique that generates a signal indicating a signal level of a radio signal from the radio signal and displays the signal level on a display device is disclosed in Patent Literature 2. Patent Literature 2 relates to an electromagnetic wave monitoring device, which includes a transmission line that is connected to an appropriate radio device and an electromagnetic wave generation device 1 and receives or transmits a specified electromagnetic wave and a coupling means that is magnetically coupled to the transmission line and converts the electromagnetic wave received or to be transmitted into an electrical signal such as a voltage signal or a current signal, for example. Further, the electromagnetic wave monitoring device described in Patent Literature 2 includes a detection means that is connected to the coupling means and detects the electrical signal output from the coupling means, a storage means connected to the detection means, and an annunciation means connected to the storage means through a switching means.
Further, the technique that stores the energy of a transmission signal in a capacitor that functions as an energy storage unit and generates DC voltage from the energy stored in the capacitor is disclosed in Patent Literature 3.
Further, the technique that generates high voltage from a received signal is disclosed in Patent Literature 4. Patent Literature 4 relates to a portable receiver that includes a reception antenna, a specific electric wave detector circuit, a power supply control circuit, a load circuit and a battery. In Patent Literature 4, the specific electric wave detector circuit is composed of a tuned circuit in which a parallel resonance frequency is adjusted to catch a specific electric wave only and a high voltage generator circuit that outputs detection voltage generated by stepping up the voltage obtained in the tuned circuit three times or more. Then, in Patent Literature 4, the power supply control circuit 3 turns off a switching element and does not supply power to a load circuit in the standby mode where the detection voltage is low, and turns on the switching element and supplies power to the load circuit in the specific electric wave receiving mode where the detection voltage is high.